1. Field of the Invention
The present invention relates generally to a glass sheet heating system and more particularly to an improvement in an apparatus for locally heating selected portions of glass sheets at a higher temperature than the rest of the same.
2. Description of the Related Art
Shaped or curved glass sheets are widely used as windshields as well as rear windows in automotive vehicles. Manufacture of such shaped glass sheets typically includes heating flat sheets to the softening points of the glass, shaping the heated sheets to a desired curvature, and then cooling the bent sheets to a temperature below the annealing range of the glass. The last step or cooling should be swiflty effected to temper the glass sheets to increase the mechanical strength as well as the resistance to breakage. This tempering also causes the glass sheets to fracture into relatively small and smoothly surfaced fragments, rather than jagged fragments, upon damages resulting from impact.
Flat glass sheets must be shaped to precisely defined curvature dictated by the shape and outline of the frames of vehicle windows, and it is often required that some portions of the glass sheets be bent with a much smaller radius of curvature than that of the remaining portion of the glass sheets. To meet this requirement, it is necessary to heat some particular portions of the glass sheets at a higher temperature than the rest of the same.
U.S. Pat. No. 3,273,988 issued to Dean et al discloses stationary auxiliary burners positioned to provide localized heating of the peripheral edges of glass sheets. The only type of localized heat is, however, a strip along the direction of conveyance of the glass sheets.
Japanese laid-open patent publication No. 54/60314 discloses an apparatus for bending glass sheets. The disclosed apparatus includes a furnace in which burners are suspended for reciprocating motion to locally heat straight side edges of a sheet of glass placed on a bending mold. The localized heat is thus limited to two parallel straight lines.
U.S. Pat. No. 4,441,907 issued to Nitschke discloses an apparatus for providing localized heating of the curved edges of a glass sheet. The patented apparatus includes a pair of elongated gas burners positioned to locally heat designated portions of glass sheet as they are heated while being conveyed through a furnace on a horizontal roller conveyor. These localized heaters are mounted on a carriage driven by a drive mechanism. The drive mechanism causes the carriage to move in a direction parallel to the direction of conveyance of the glass sheet in order to provide continuous heating of the designated portions of the glass sheet between an upstream position and a downstream position of the carriage. At this time, the carriage is driven downstream in unison with movement of the glass sheet. After the carraige reaches its downstream position, the drive mechanism causes the carriage to return to its upstream position in preparation for another cycle. There are a few disadvantages to this particular localized heater. First, there must be formed at one side of the furnace a relatively large horizontally extending opening through which the localized heaters are connected to the carriage movable along the exterior path. Formation of this opening results in a great amount of heat loss within the furnace. Secondly, the carriage has to be returned to its original or upstream position for the next cycle after it reaches its downstream position. This reciprocating movement of the carriage will substantially slow down the operation cycle of thermal treatment to the glass sheet.